The jet grouting technique consists in breaking up the ground by means of a jet of fluid having very high kinetic energy that is delivered in a borehole, the jet of fluid eroding the soil in which it is desired to make an excavation. To form the jet, a nozzle is used which is fixed to the end of drilling rods, these rods serving both to convey the fluid under high pressure to the nozzle(s) and to move the nozzle progressively into the ground. More precisely, the nozzle(s) is/are mounted on a member usually referred to as a “monitor” or an “injection head” which is fixed to the bottom end of the drill string, said monitor itself possibly being fitted at its own bottom end with a mechanical drilling tool. As is known, the fluid that is usually used is a cement-based grout which makes it possible, after boring, to make a foundation element of cement that is molded in place in the ground. It is also possible to have a plurality of jets of fluid, one of which may be a gas such as air.
In the description below, mention is made of liquid, but it should be understood that in special cases, the fluid could be constituted at least in part by a gas.
The liquid is conveyed by the rods and it is delivered from the surface by a pump at pressures lying in the range one to several tens of megapascals (MPa). The inside diameter of the rods conveying the liquid needs to be large enough to limit head losses in the rods. This diameter may typically be of the order of 20 millimeters (mm) to 50 mm. In contrast, the nozzle outlet diameter needs to be small enough to impart sufficient speed to the outgoing liquid jet to enable it to erode the ground remotely. Typically, the outlet diameter of the nozzle lies as a general rule in the range 2 mm to 5 mm, and the outlet speed of the liquid from the nozzle is one to several hundreds of meters per second (m/s).
In order to obtain a high quality jet, it is desirable for the inside shape of the nozzle to be optimized in order to conserve as high a speed as possible for the liquid jet as the jet departs from the nozzle going towards the ground so as to enable it to erode the ground as much as possible while using a minimum amount of kinetic energy. Optimized nozzle shapes that satisfy this requirement are in widespread use.
However, even with such nozzles, it is found that the jet quickly loses effectiveness in terms of its ability to erode the ground, such that considerable amounts of kinetic energy are required, so that when the rods are moved in translation, and possibly also in rotation, the ground is eroded at a considerable distance from the nozzle, for example at a distance of several decimeters (dm). The active radius of the jet of liquid under pressure for forming a column, a column sector, or a flat element generally remains mediocre, lying in the range a few decimeters to 1 or 2 meters (m) depending on the method implemented, the nature of the ground, and the energy used.
In order to increase the effectiveness of the jet, proposals have been made, in particular in U.S. Pat. No. 5,228,809, for an embodiment of the injection head or monitor that enables the quality of the jet to be improved.
Accompanying FIG. 1 shows the injection head described in that patent. The injection head 10 comprises a body 12 having a side wall 14 defining an internal cavity. A nozzle 16 for injecting liquid under pressure is mounted in the outside wall 14 of the monitor. In this figure, there can also be seen elements 18 for coupling to the string of rods and elements 20 and 22 for coupling to the pressurized liquid pipe and to an annular air pipe extending along the rods in order to feed simultaneously the nozzle with liquid and an annular nozzle with air. According to that patent, the nozzle 16 is fed from the pressurized liquid pipe 22 by a passage 24 made in the monitor body, and by a tube 26 connecting the end of said passage to the inlet of the nozzle 16. The tube 26 is of constant section and of regular curvature so as to limit disturbance to the pressurized liquid between the drilling rod and the nozzle 16 itself. Nevertheless, as explained, the diameter of the injection nozzle is very small compared with the diameter of the pipe used for conveying the pressurized liquid along the rods. The technique described in that above-mentioned United States patent is therefore not completely satisfactory, in particular because of said differences in section. Furthermore, it can be seen that the passage 24 forms right angles relative to the pipe 22 and the pipe 26. Such a disposition leads to significant disturbances in the flow of liquid at the inlet to the tube 26, and thus in the nozzle.
In addition, the monitor described in that patent does not enable the mechanical tool that might be mounted on the monitor to be fed with liquid under pressure.
Unfortunately, it is necessary to feed the mechanical tool with drilling liquid in order to lubricate it, and above all in order to raise the cuttings that result from the drilling.
It is therefore advantageous to be capable of having a single monitor suitable both for feeding jet grouting nozzles under good conditions and also for feeding the drilling tool at the bottom of the monitor. In order to obtain optimum feed to the injection nozzle, it is necessary in some cases to be able to interrupt the feed to the drilling tool so that the entire flow of fluid, e.g. a grout under high pressure, is used for feeding the injection nozzle.